In a wire-cut electric discharge machine, a train of successive voltage pulses is applied across a machining gap between a travelling wire electrode and a workpiece being machined to induce electric discharges across the machining gap, thereby resulting in the machining of the workpiece. The wire electrode is continuously transported through the gap during machining while a dielectric fluid, such as water having a resistivity maintained at a generally constant value of more than 10000 ohm-cm, is supplied to the gap.
In general, when machining a workpiece using the wire-cut EDM, voltage pulses are usually applied through a pair of electrically conductive current pickups across a regularly poled machining gap, i.e., one in which the wire electrode is negative and the workpiece is positive. The current pickup, which is typically made of a hard, highly conductive, wear-resistant metal, for example hard alloys, such as silver tungsten and tungsten carbide, is always in physical contact with the travelling wire electrode to deliver thereto machining current pulses having a period of, for example several .mu. sec. During machining, the travelling wire electrode rubs on the current pickup, thereby generating frictional heat, which may result in wire breakage or dull the leading edge of the machining current pulse. In order to cool the current pickup, a dielectric fluid, the temperature of which is maintained at a generally constant value, is usually supplied to it.
When voltage pulses are repeatedly applied across a regularly poled machining gap, i.e., one in which the wire electrode is negative and the workpiece is positive, having a water-based dielectric fluid in the gap, the dielectric fluid may become electrolyzed resulting in an electrolytic current flowing across the low-resistivity portion(s) of the gap. This may, in turn, result in the formation of an affected layer on the workpiece surface. Particularly when machining a workpiece made of a so-called super hard alloy, for example cemented alloys such as cemented tungsten carbide which is obtained by sintering fine tungsten carbide powders mixed with a small amount of cobalt binder, or other materials susceptible to electrolysis, the (cobalt) binder material, which has a relatively high electrolytic solubility, may even dissolve out of the workpiece.
In Japanese Patent Publication No. 63-17569, a power supply for a wire-cut EDM is disclosed which has a main power source for generating a discharge across the machining gap with the wire electrode being negatively poled and the workpiece being positively poled, i.e., of regular polarity, and an auxiliary power source for generating a discharge across the gap with the wire electrode being positively poled and the workpiece being negatively poled, i.e., of reverse polarity. In this apparatus, a voltage of reverse polarity from the auxiliary power source is first applied across the gap to initiate a discharge. After detecting the occurrence of a discharge, the voltage from the main power source, of regular polarity, is used to maintain the discharge while power from the auxiliary power source is interrupted. As a result, the undesirable effects of the electrolytic activity are decreased.
FIG. 5 depicts a power supply illustrative of this prior art. The power supply comprises a power source V1 for delivering a high voltage at a large current, a power source V2 for delivering a low voltage at a small current, a large-capacity condenser C1, and a small-capacity condenser C2. The power supply further comprises transistors TR1 and TR2 for controlling current, flow pulse controllers G1 and G2, charging resistances R1 and R2 for the capacitors C1 and C2, respectively, and potential dividing resistances R3 and R4. A wire electrode W, unwound from a supply reel 2, is taken up by a take-up reel 4 while electric discharges are repeatedly induced between a workpiece 8 and the travelling wire electrode W to machine the workpiece 8. The wire electrode W is guided between wire guides 6. An upper current pickup 10A and a lower current pickup 10B, both of which are in physical contact with the travelling wire electrode W, deliver current to the travelling wire electrode W. Further, fluid guides 12 are used for guiding a dielectric fluid (not illustrated) to the machining gap formed between the wire electrode W and the workpiece 8.
Operation of the apparatus shown in FIG. 5 will now be described with reference to FIGS. 6(a) to 6(d). FIGS. 6(a) and 6(b) are respectively illustrative of waveforms of the gap voltage, -Vg, developed in the machining gap, and the current I flowing across the machining gap. FIGS. 6(c) and 6(d), respectively, illustrate the ON/OFF timing of the transistors TR2 and TR1. As illustrated in FIG. 6(c), a voltage is first applied from the low-voltage power source V2 in reverse polarity, i.e., with the wire electrode W positively poled with respect to the workpiece 8, by turning on the transistor TR2 to initiate a discharge. Then, when the occurrence of a discharge is detected, the transistor TR1 is turned on, as illustrated in FIG. 6(d), to thereby apply a voltage across the machining gap from the high-voltage power source V1 in regular polarity, i.e., with the wire electrode W negatively poled with respect to the workpiece 8. This voltage maintains the discharge while the power from the low-voltage power source V2 is interrupted.
When machining with such a change in polarity of voltage pulses, electrolytic effects on the surface of a workpiece tend to be inhibited. However, in this case, as compared to machining using regular polarity voltage pulses only, the current pickups 10A and 10B may be subjected to electrolytic corrosion during reverse polarity operation since the dielectric fluid for cooling the current pickups may become locally electrolyzed. This electrolytic corrosion of current pickups may result in an increase in an electrical contact resistance and frictional resistance, which in turn may result in wire breakage. Therefore, the current pickups 10A and 10B will require frequent replacement in order to prevent wire breakage.